1) Field of the Invention
The present invention relates to technology for a directivity control apparatus and directivity controlling method. In particular, the invention relates to technology suitable for controlling directivity of an array antenna by means of adaptively updating a weighing coefficient with respect to each antenna signal of an array antenna.
2) Description of the Related Art
An array antenna is a scheme in which multiple antennas receive signals, and for each antenna, a weighting coefficient (weight) is given to a reception signal to separate signals from a specific direction.
As shown in FIG. 7, for example, signals received through multiple (four in FIG. 7) antennas 100 are multiplied, by the multiplier 101 for each antenna 100, by weighting coefficients w0, w1, w2, and w3. These weighting coefficients w0, w1, w2, and w3 are updated (controlled) by the weight updater 104 on the basis of a difference (error) e which is obtained by the adder 103 by comparison of a reception signal having been added (combined) by the adder 102 with a pilot signal that is an already known reference signal.
As a result, effects and benefits such as that a gain is raised in an specific direction, or interference is eliminated by aiming null at in a specific direction. In this instance, the distance among the antennas 100 is often used 1 wavelength or 0.5 wavelength of a reception signal (or transmission signal).
FIG. 8 shows an example of directivity of an array antenna using a certain weight. From FIG. 8, the array antenna has strong directivity (or null points) in six directions. In this manner, giving directivity is called beam forming.
(2) Adaptive Array Antenna
When the direction of the target is already known, it is easy to set a weighting coefficient. For example, the method of using GPS (Global Positioning System) can be usable. However, in mobile communication, the direction of the target is normally unknown. Thus, a weighting coefficient is set using algorithm such as NLMS (Normalized Least Mean Square).
Here, referring to FIG. 9, a specific operation (updating of a weighting coefficient) if an adaptive array antenna using the NLMS method is explained. Simply, an already known pilot signal (pattern) is compared to a reception signal to obtain an error e. In order to make the square of the error e become the minimum, a weighting coefficient wm [m represents the number of the antenna, m=0, . . . , M−1 (M is an integer not smaller than 2)] is changed.
That is, when a reception signal of each antenna 100 is Xm, and a signal obtained after beam forming (that is, an output signal of the adder 102) is y, y is expressed by the following formula (1.1)y=Σwmxm  (1.1)
Further, as to a pilot signal, as shown in the following formulas (1.2) and (1.3), a normalized coefficient P and the amplitude A of a reference symbol are calculated.
                    P        =                              ∑                          m              =              0                                      M              -              1                                ⁢                                                                  x                m                                                    2                                              (        1.2        )                                A        =                                            1              M                        ⁢                                          ∑                                  m                  =                  0                                                  M                  -                  1                                            ⁢                                                                                      x                    m                                                                    2                                                                        (        1.3        )            
Further (for example, by normalizing a channel estimation value ξ), a phase term Φ is obtained by the following formula (1.4).
                    Φ        =                  ξ                                  ξ                                                          (        1.4        )            
As described above, the error e is obtained by the following formula (1.5). Here, d expresses a pilot pattern.e=AΦd−y  (1.5)
In FIG. 9, an arithmetic operation by this (1.5) is realized by means of the multipliers 105 and 106 and the adder 103. The weight updater 104 updates the weighting coefficient using this error e. When the current weighting coefficient is wm, and when the updated weighting coefficient is wm′, a new weighting coefficient wm′ is obtained by the following formula (1.6).wm′=wm+(μ/P)exm  (1.6)
In the formula (1.6), μ is a step factor. In FIG. 9, the present formula (1.6) is realized by the multipliers 141, 142, and 143, the adder 144, and the weighting coefficient holder 145. That is, a reception signal xm of each antenna 100 is multiplied by an error e, a normalized 1/P, a step factor μ by means of the multipliers 141, and 142, and 143 to obtain the second term [(μ/P) exm, to which the current weighting coefficient wm [the first term of the above formula (1.6)] held in the weighting coefficient holder 145 is added by the adder 144. As a result, a new weighting coefficient wm′ is obtained.
In this manner, as the weighting coefficient wm is updated, the beam direction is changed to the direction such that the error e becomes zero, as shown in FIG. 10(A), for example. Accordingly, when the step factor μ is made to be “1” (when effect by noise is absent), a beam turns to the target by one step. However, effects of noise cannot be disregarded. Here, the above formula (1.6) converges when the above formula (1.6) is 0<μ<1. Hence, as shown in FIG. 10(B), the beam direction is controlled in such a manner that the error e after one step turns to be (1−μ)×error e.
Here, if the step factor μ is large, conversion is fast, but is subjected to effects of noise etc. Thus, it converges while vibrating largely. In addition, after the convergence, the beam direction is fishtailed and unstable, and as a result, the error e becomes large. Thus, a considerable small value is normally set to the step factor μ. However, if the step factor μ is small, convergence is delayed.
That is, in a case of using NLMS algorithm, if the direction of the target does not move, it converges at some future time. However, when using in mobile communication, it is necessary to track a moving target (mobile terminal). When the target is fixed, it becomes insensitive when the step factor μ is small so that an error rate becomes low, but since conversion is delayed, it becomes impossible to track the target which is being moving in high speed.
Thus, in the present situation, the step factor μ is adjusted in consideration with trade off between the speed of convergence and the largeness of an error e after convergence. That is, it can be said that NLMS is not an algorithm generally suitable when the target moves. Hence, an algorithm which can support a fixed target and a target which is moving in high speed is desired.
In this instance, the following patent document 1 proposes the previous art in which prediction is performed using information in past, and the beam direction (directivity) is controlled.
In this technique, a reception signal (reception response vector) is divided into frequency components (signal wave form for each frequency) . By means of making time t large, changes of the signal form of each frequency is predicted, and by means of combining the signal forms at that time, a reception vector at time t is estimated. As a result, it is prevented that an error is caused by the effect of fading, as in the case of extrapolation, so that a transmission response vector can be estimated. Accordingly, even if the terminal moves faster, and the degree of fading is extreme (in the circumstance when a Doppler frequency is high), it is possible to accurately control transmission directivity.
However, it can be said that this technique is a significantly rough method from the view of directivity control. That is, in the previous art, a reception signal is divided into a signal wave form for each frequency, and its coefficient is obtained. By means of predicting changes in the signal wave form for each frequency, and combining the signal wave form at that time. In this manner, a transmission response vector is estimated by estimating a reception response vector. Thus, precise directivity control cannot be expected regardless of the number of antennas. That is, when the number of antenna is increased, and the beam form for each antenna becomes small, it can become impossible to accurately track the target.
In addition, it is necessary that a transmission weight vector is calculated by using an algorithm such as Winner solution from the estimated transmission response vector. Thus, a great amount of calculation is needed to obtain the final transmission weight vector (weighting coefficient). As a result, it can become impossible to track a target which is moving in high speed.
With the foregoing problems in view, it is an object of the present invention to make it possible to perform detailed directivity control of an antenna beam with a small amount of calculation in an array antenna, and also to make it possible to accurately track a target which is moving in high speed.
[Patent Document 1] Japanese Patent Application Laid-Open No. 2003-32167